The Saint-Honoré carbonatite REE zone, Québec, Canada: Combined magmatic and hydrothermal processes.

English Abstract:

A new model is proposed for the genesis of rare earth element (REE) mineralization at the Saint-Honoré alkaline complex. The originality of the proposed model is to combine both magmatic and hydrothermal crystallization of REE-bearing minerals. Actually, most of the REE mineralization models in carbonatites are based on a late stage hydrothermal activity, where REE are leached from primary minerals such as dolomite and apatite and subsequently deposited in late stage units. Such a model is in agreement with observations of the upper Fe-carbonatite REE mineralisation of Saint-Honoré alkaline complex. It shows characteristic textures of hydrothermal mineralization: polycrystalline clusters hosting bastnaesite crystallized radially from carbonate or barite crystals, the presence of halite and silicification within strongly brecciated units. However, in the deeper part of the Fe-carbonatite (below 1000 meters), bastnaesite inclusions in primary barite suggest an early magmatic crystallization of REE minerals before hydrothermal leaching. It is interpreted that hydrothermal activity was weaker in the deeper part (Cl-depleated, a progressive decrease with depth of secondary fluid inclusion trapped in carbonates, brecciation is significantly less intense), hence preserving the magmatic textures. This magmatic crystallisation of rare earth minerals could be an important factor in generating high volume world-class REE deposits. An example of magmatic world-class deposit is Mountain Pass in California where idiomorphic bastnaesite crystals were described. Furthermore, crystallization of primary barite could be an important exploration guide for REE in carbonatites because its presence seems to reflect favourable conditions to crystalize primary REE minerals.